The present invention relates to a rotary blade for pruning machines, for example, lawn mowers. More particularly, the present invention is directed to a rotary blade for engine-driven or motor-driven pruning machines.
Pruning machines which use a metal blade and are driven by an internal combustion engine or an electric motor are well known in the art. The blade is rotated substantially parallel to the ground, and the machine can be freely pushed to any area to be pruned with the use of very little manpower and is efficient for cutting in comparatively narrow spaces. These well known devices can be used to cut grass and weeds, whether tall or short or tough or fragile. However, unfortunately, because of the high speed of rotation of the cutting blades in these devices, up to about 6,000 rpm or more, it can be extremely dangerous not only for the operator per se but also for persons in the vicinity of the cutting operation. For example, there have been many cases where the operator's foot has accidentally slipped under the housing and has been struck by the rotating blade, causing serious injury. Moreover, if the blade accidentally contacts any rigid obstacles, it may counteract and strike the operator himself or any other person nearby, causing serious injury. Thus, it is not unusual that a broken portion of the blade will be discharged from the device, causing injury to a person standing nearby. Furthermore, even in the case where no obstacles are present, there still exists the danger of operator mistake.
In this connection, there have been proposed certain flat, bar-type blades made from high molecular weight elastomeric material, such as polyurethane elastomers, according to U.S. Pat. Nos. 3,343,350, 3,623,305 to Freedlander e al. However, such flat, bar-type blades are only effective for cutting short and fragile grasses but are ineffective for cutting thick ligneous weeds, such as Ambrosia artemisilifolia L. (Bitter weed) or Solidago alti (ssima L.) because the cutting of such ligneous weeds violently shock the operator when they are cut down due to the comparatively long blade length and their intermittent cutting action. Moreover, if a portion of the blade becomes broken away as a result of a collision with any rigid obstacle, such as a hidden tap, iron post or rock, a violent vibration will occur due to an imbalance created in the blade structure. This makes the device inoperable. The present invention is intended to eliminate the above defects which are known to exist in elastomeric flat, bar-type blades.
An object of the present invention is to provide a novel elastomeric rotary blade for pruning machines which is effective in cutting the thick ligneous stalks of the weeds.
Another object of the present invention is to provide a novel, elastomeric rotary blade which is easy to care for and provides safety in operation.
A further object of the present invention is to provide a rotary blade which is seldom broken when it collides with a rigid obstacle.
Still another object of the present invention is to provide a blade which can be positively operated, regardless of obstacles, in an area where many visible obstacles are present.
Yet another object of the present invention is to provide a novel, long-life blade which can be readily removed from the pruning device when the blade becomes worn.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
As result of a long study and many experiments, it has been found that a rotary disc-type blade having a plurality of blade bodies radially projecting from its periphery will give far less shock to the operator when compared with known bar-type blades, when the blades collide with a rigid obstacle. It has furthermore been found that this shock is also substantially reduced if a plurality of holes are provided along the periphery of the disc portion or supporting member of the blade. Thus, narrow blade bodies are more easily bent than are the broader, bar-type blade bodies and this bending occurs more readily if a large round hole is disposed nearby. Because of the extraordinary high speed of rotation, up to about 6,000 rpm, even if the first blade which contacts a rigid stalk of weed is bent, the next blade instantly contacts the stalk damaged by the first blade body and, consequently, smoothly cuts down the stalk with little shock being transferred to the operator. Such a result was unexpected.
One of the characteristic features of the present invention is the use of a high molecular weight elastomeric material as the sole principal material of which the blade, including the blade bodies, is made. Suitable elastomeric materials which can be used include natural rubber, styrene-butadiene rubber, polychloroprene, cis-1,4-polyisoprene, cis-1,4-polybutadiene, syndiotactic 1,2-polybutadiene, ethylene-propylene copolymer, ethylene-propylene rubber, butyl rubber, polyurethane polyester elastomers, Hypalon.sup.R (CSM), polyvinyl ethers, polypropylene oxide, epoxy resins including polyepichlorhydrine, polystyrene, polybutadiene block copolymers, butadiene-ethylene copolymer, isobutylene-isoprene copolymer, chlorosulfonated polyethylene, organic polysulfides, acrylic rubbers, and ethylene-vinyl acetate copolymer.
In consideration of the objects of the present invention, it is preferable that the material used, if possible possess, good tensile strength, good tear resistance, good abrasion resistance and good flexibility. Also, since the blade is often exposed to strong sunlight, especially in summer, it is preferable that the elastomer possess good light stability.
The preferred elastomers include natural rubber, polyisoprene, polychloroprene, polyurethanes or polyesters and the like. Although polyurethane elastomers and polyester elstomers tend to elongate due to tension, they are particularly effective materials for the practice of the present invention.
Materials other than polyurethanes or polyesters which are somewhat less useful in view of their mechanical properties, namely a comparatively poor tear strength and abrasion resistance, and/or large elongation, can be substantially improved up to the same degree with those of the polyurethanes or polyesters by admixing thereinto some reinforcing agents or reinforcing materials such as silica (amorphous silicone dioxide), carbon black, staples of synthetic, carbon or glass fibers, cat whisker or the like. Accordingly, it should be understood that the above recommendation with respect to polyurethanes or polyester elastomers relates to the best sole material which can be used without the presence of any reinforcement material. When using polyurethanes and polyesters, a reinforcing agent such as glass fibers may be used to improve the elongation of the polyurethanes or polyesters.
It is possible to improve the heat durability, the abrasion resistance or the creep resistance, or to lower the cost by blending the elastomer with the usual synthetic resins such as polyvinyl chloride, acrylonitrile-butadiene-styrene polymer, polyolefins, ethylene vinyl acetate copolymers, acrylonitrilestyrene copolymers and the like or special resins such as acetal resins, silicone resins, fluorine-containing resins and the like. Generally speaking, the blend of other resins into the urethane resin (elastomer) inherently reduces the elasticity of the urethane resin, but, on the other hand, makes it easier to resharpen the blade. Particularly the use of a small amount of a fluorine-containing resin, preferably a perfluoro resin such as polytetrafluoroethylene is noteworthy because of its marked effect on the coefficient of friction.
When injection molding is used for the production of the blade of the present invention, the amount of the additive reinforcing materials such as glass fiber staples which may be added into the elastomer base material, will be limited due to the effect on the fluidity of the basic resin. When thermoplastic-type polyurethanes are utilized, the additive can be added in an amount of about 15 to 30% by weight based on the weight of the resin and depending on the grade of the resin and the temperature of the operation. Similarly, the maximum length of the additive staples is up to about 15 mm.
The novel blade of the present invention has a good sharpness which has the same effectiveness in cutting thick, ligneous stalks of weeds as the conventional steel blades but is far superior in this regard to known flat bar-type elastomer blades and can efficiently and smoothly achieve the desired cutting operation without material shock to the operator's hand.
During the testing of the blade of the present invention, it was discovered that the engagement by the binding nut which secures the blade to the drive shaft will sometimes become loose due to the operation of the device. One of the factors which causes this phenomenon is the creepness and the stress relaxation of the material being utilized. When the blade is continuously held to the drive shaft for a long time under the strong compression of the screw, a semi-permanent distortion in the blade may occur. Then, as the stress is lowered, the resilient power to the binding nut becomes lower and, thus, the screw may eventually become loosened. However, the direct cause of this loosening effect is perhaps the centrifugal force and the radial elongation of the blade caused thereby. The relaxation phenomenon is substantially caused by the large centrifugal force, greater than 13,000 G's, due to the rapid rotation of the blade. Naturally, this force will gradually decrease toward the center of the blade, in proportion to the decrease of the radius, but the blade will be radially stretched with this large force, and concurrent with this elongation, the thickness of the blade will be reduced, even at its central mounting hole nearby. Thus, to prevent such elongation and to achieve commercialization of the elastomeric rotary blade of the present invention, the following techniques are utilized:
(a) A core made of a rigid material is inserted into the area surrounding the mounting hole;
(b) A cover made of a rigid material and connected with the supporting member is installed over one or both surfaces of the supporting member surrounding the mounting hole;
(c) A cloth or a net-type material having a high Young's modulus is inserted into the supporting member surrounding the mounting hole;
(d) A linear material having a high Young's modulus is disposed so as to radially surround the mounting hole;
(e) The central portion of the supporting member is made of a hard high molecular weight elastomer and the outer circumference is made of a more flexible, high molecular weight elastomer;
(f) A device comprising any combination of the above techniques (a) to (e).
The term "blade body" as discussed above means the blade per se or the blade including the neighboring elements thereof. Thus, the term "blade" includes the blade body and the disc-like supporting member.
With respect to the techniques (a) to (f) above, (a) is the one most generally used. The core can easily be positioned in the disc by any insert molding technique when injection molding is utilized, or by a two-step pouring method of the raw material into dies when compression molding is applied thereto. As to the type of core material, a steel disc having a thickness of about 1 mm is frequently used, but any rigid material such as aluminum or titanium or an engineering type of plastic may also be used in place of a steel core. Advantageously, a plurality of small radial or circular holes are provided in the blade in the vicinity of the core so as to expose both the upper and the lower core surfaces to the environment throughout these holes. However, if the core per se is compatible with the high molecular weight elastomer utilized, these holes can be reduced in number or omitted.
The diameter of the core may be from about one-third to about one-half of that of the blade. If the diameter is greater than this amount, the flexibility of the disc (the supporting member) at its marginal portion will be reduced.
According to the (b) technique, the molded blade is covered with a rigid material at its central portion. The shapes and the sizes of the covers may be designed as in the device (a). The cover may be provided on one side surface of the blade, but it is more preferable to cover both side surfaces of the blade. The cover must be closely connected with the disc. To provide this connection, any appropriate method such as rivetting (when the covers exist on both sides of the blade) or pinning (when the cover exists on one side of the blade) is usually applied. However, these illustrations are merely illustrative. Other devices include means wherein a pair of washers having elongated sleeves are joined together so as to make an intimate connection as well as the formation of a central mounting hole as hereinafter described.
The (c) technique may be realized by utilizing insert molding or compression molding. The cloths usually inserted into the supporting member are woven polyamides, polyethylene terephthalate or glass fibers. If desired, a steel net electrically plated with copper may be utilized. Generally, any synthetic fiber which has a high affinity to high molecular weight elastomers can be utilized. However, the affinity of glass fiber to high molecular weight elastomers is comparatively low due to attached machine oil, etc. Because of this, an activation treatment such as borane or a silicone pretreatment is preferable for the glass fiber.
A canvas inserted according to this means never adversely affects the flexibility of the blade and, accordingly, it is possible to extend the diameter of the inserted canvas up to the peripheral circles or even through the blade body per se. The inserting method can be carried out manually on an industrial scale similar to the hand lay-up method used in fiber reinforced plastics by utilizing dissolved high molecular weight elastomers in an appropriate solvent or by utilizing low molecular weight liquid elastomers insufficiently polymerized together with a catalyst.
The technique may be carried out similarly as in the technique (c). In this method a reinforcement made from linear materials which are combined concentrically and radially to form said reinforcement are charged into a metal mold and then compressed. As usable linear materials, there can be included tire cord, glass fibers, yarn, carbon fiber, piano wire, or phosphor bronze wire, etc. The last two materials have the advantage that the wiremade reinforcement is so tough but flexible that it can be easily put into the mold and, furthermore, the molded product (the blade) is sufficiently reinforced up to its blade body without losing its own flexibility. Also, in this case, an activation treatment, as in the (c) technique, will be preferred.
The (e) technique can be utilized with the same kind of high molecular weight polymers (elastomers). Therefore, there is little problem as to compatibility at the interface zone. However, a substantial tension will exist at the interface between the central hard elastomer and outer soft elastomer. Thus, it is desirable that high tensile fibers or whiskers be provided to radially traverse over the interface. The elastomer at the center of the supporting member can be hardened by means of curing, adding thereto a large amount of filler such as carbon black, silica, etc., or by blending with glass fiber staples.
The above five techniques, as explained above, may be used solely or in combination. For example, to extend a radial reinforcement made from rigid wire, from piano wire or from phosphor bronze wire (or beryllium bronze wire) toward the outer direction so as to form thorns and to bury their point portions into the blade body can be regarded as a combination of techniques (a) and (d). This is a useful embodiment of the present invention. Moreover, this thorn-like reinforcement can be modified with concentric rings made from the same kind of wire so that a framework can be prepared. This framework can easily be put into a die on the molding.
As shown by the attached drawings, the blade bodies of this invention may take several shapes. A rectangular blade body as shown in FIG. 1 is the most standard type. If the blade bodies are somewhat inclined to the radius direction as shown in this figure, it becomes possible to change the direction of the acting blades depending on the kinds and density of the weeds being pruned away. FIG. 2 shows a rotary saw-type blade; FIG. 3 shows a concaved blade and FIG. 4 shows a blade having an involuted shape. However, whatever shape of the blade body is utilized, it is desired that a plurality of blade bodies project approximately radially from the circumference of the disclike supporting member.
The blade bodies of the present invention, made from high molecular weight of elastomers, are very tough against abrasion, but they can be easily resharpened with a grinder. However, if the inner walls of the buffering holes are sharpened so as to form a sharp edge, it is very convenient to recover the worn blade. In order to recover by cutting the outer portion of the blade away, proper marking, such as the use of double-relief concentric circles, may be provided at the desired cutting line, as shown hereinbelow. The cutting line should lie so as to cross with the buffering holes.